Summary While other studies in the Radiation Biology Branch have shown nitroxides to be efficient antioxidants and radiation protectors, recent studies have shown that they can be used as functional MRI contrast probes. Because nitroxides are paramagnetic their presence in tissue can be monitored non-invasively by MRI. Further the disappearance of nitroxide induced MR intensity enhancement in tissue is a result of intracellular reduction of the nitroxides to the hydroxylamine. By following the rate of reduction of the nitroxide in tissue the redox rate can be determined. This property distinguishes nitroxides as functional MR contrast agents revealing information about the intracellular redox capacity of cells/tissues. Nitroxide-mediated MRI was recently used to provide additional insight into the radio-protective properties of nitroxides, particularly with respect to the differential radioprotection of Tempol toward normal tissues as opposed to tumor. Tempol administered 10 min prior to fractionated radiation treatment does not protect SCC and HT-29 tumor growth; whereas, Tempol administered in the same fashion protects against radiation-induced salivary gland damage. Our lab has made a recent major advancement that can be used to explain this difference. We have shown that Tempol is reduced in tumor tissue to the non-radioprotective hydroxylamine approximately 2-fold faster than in salivary gland tissue. Thus, the radioprotective form of Tempol is still present in salivary glands at the time of radiation to provide protection. Nitroxide based MRI studies thus have the potential to guide to appropriate timing of nitroxide administration to yield maximal radioprotection of normal tissues without protection of tumor. In collaboration with the Radiation Oncology Branch a Phase I clinical trail (first in humans) has been submitted to evaluate Tempol as a functional MRI contrast probe. Further studies have shown that 6-membered ring nitroxides are reduced faster in tissue than 5-membered ring nitroxides. We have also shown that the reduction rate of nitroxides differs among tumor types and among various normal tissues in mice and that reduction rates in tumors correlate with the extent of tumor hypoxia. Since nitroxides readily penetrate cell membranes and are potent antioxidants, they may be of use in other areas of medical research such as ischemia/reperfusion injury studies, stroke, prevention of cataracts, inflammatory processes, and aging. Nitroxide based MRI evaluation may have clinical utility in defining the above-mentioned conditions.